Process for plating articles with silver-palladium alloys

ABSTRACT

A PROCESS FOR PLATING AN ARTICLE WITH A SILVER-PALLADIUM ALLOY COMPRISES THE STEPS OF: PROVIDING THE ARTICLE WITH A CONVENTIONAL SILVER STRIKE, ELECTROPLATING THE ARTICLE IN A SILVER CYANIDE SOLUTION AT A CURRENT DENSITY OF ABOUT 10 AMPERES PER SQUARE FOOT FOR ABOUT 20 MINUTES, ELECTROPLATING THE ARTICLE IN AN AGITATING PALLADIUM CHLORIDE AND AMMONIUM CHLORIDE ACIDIC SOLUTION AT A CURRENT DENSITY OF ABOUT 4 AMPERES PER SQUARE FOOT FOR ABOUT 8 HOURS, AND HEATING THE ARTICLE AT A TEMPERATURE OF ABOUT 300 TO 400*C. FOR ABOUT 8 HOURS TO ALLOY THE SILVER AND PALLADIUM THEREON.

April 1973 H. J. WESOLOSKI 3,725,219

PROCESS FOR PLATING ARTICLES WITH SILVER-PALLADIUM ALLOYS Filed May 4,1965 Sim/OWE United States Patent Ofiee 3,725,219 Patented Apr. 3, 19733,725,219 PROCESS FOR PLATING ARTICLES WITH SILVER-PALLADIUM ALLOYSHenry J. Wesoloslri, Milton, Masa, assignor to Allis- ChalmersManufacturing Company, Milwaukee, Wis. Filed May 4, 1965, Ser. No.453,009 Int. Cl. C23b 5/52 U.S. Cl. 204-37 R 1 Claim ABSTRACT OF THEDISCLOSURE A process for plating an article with a silver-palladiumalloy comprises the steps of: providing the article with a conventionalsilver strike; electroplating the article in a silver cyanide solutionat a current density of about amperes per square foot for about minutes;electroplating the article in an agitating palladium chloride andammonium chloride acidic solution at a current density of about 4amperes per square foot for about 8 hours; and heating the article at atemperature of about 300 to 400 C. for about 8 hours to alloy the silverand palladium thereon.

This invention relates to articles plated with silver-palladium alloysand processes for plating the same.

Silver-palladium alloys have the property of absorbing certain gases andare presently available in sheet and ingot form for use as getters inevacuated electrical devices such as vacuum switches and electron tubesand for use as roll-on 0r dipped protective coatings for base andprecious metals. When using this alloy as a getter to remove traces offree hydrogen and other gases in evacuated electrical devices, it wasformerly the practice to cut a small piece of the alloy from sheet stockand mechanically mount it on a support within the evacuated device. Thispractice is undesirable because of the risk of the piece breaking looseand causing a short circuit or mechanical failure. More elaboratemounting means to prevent this risk are to be avoided because additionalcosts would be imposed on an already expensive component. When used as aroll-on or dipped protective coating to provide a tarnish resistantcoating of good appearance on tableware and jewelry, expensive machineryand processes are required to bond the sheet alloy to the base memberand dip coating results in a coating of uncontrolled thickness.

It is desirable from the standpoint of cost reduction to electroplatesilver-palladium alloys on articles for gettering and other gasabsorption purposes. It is also desirable to electroplatesilver-palladium to provide tarnish resistant coatings having certaindesirable qualities as regards texture and appearance. However, it isnot known at present how to electroplate such an alloy directly sincedissimilar metals cannot ordinarily be plated simultaneously.

Accordingly, it is an object of the present invention to providearticles plated with silver-palladium alloys and to provide improvedprocesses for making the same.

Another object is to provide silver-palladium plated articles whichexhibit desirable characteristics depending on the silver-palladiumratio.

Another object is to provide improved getters for use in vacuum typedevices, particularly getters which take the form of a silver-palladiumalloy plated on parts within such devices, which parts already servesome other function within the device.

Another object is to provide a tarnish resistant plate for metalarticles, such as silver.

Other objects and advantages of the invention will hereinafter appear.

The drawing illustrates a preferred embodiment of the invention but itis to be understood that the embodiment illustrated in susceptible ofmodifications with respect to details thereof without departing from thescope of the appended claims.

In accordance with the present invention an article is first providedwith a plating of silver of desired thickness, then with a plating ofpalladium of desired thickness over the silver, and then the article soplated is subjected to heat treatment at a temperature below the fusionpoints of the article, the silver and the palladium to elfect diffusionbetween the silver and palladium to provide a plating of a truesilver-palladium alloy on the article. For purposes of illustration, theinvention is hereinafter described as applied, first, to provide agetter for a vacuum switch and, secondly, to provide a tarnish resistingplating on an article.

Referring to the drawing, there is shown a portion of a vacuum switchwhich is used in the high voltage electrical transmission art forswitching high voltage, high amperage circuits. The vacuum switch has aglass envelope 10 which is understood to be sealed at each end by meansof metal plates 12 and 14 and has a high vacuum maintained therein andan internal volumetric capacity of approximately 65 cubic inches. Plate12 affords mechanical support for an electrical connection to astationary contact support rod 16 on which is mounted a stationarycontact 18. Plate 14 is provided with aperture 20 for accommodating amovable contact support rod 22 on which is rigidly mounted a movablecontact 24. A flexible metal bellows 26 is connected in sealedrelationship as by brazing to the inner surface of plate 14 aroundaperture 20 and to the exterior or movable contact support rod 22 topermit relative movement of contacts 18 and 24 to open and closedposition as movable contact support rod 22 is moved. A hollowcylindrical sleeve 28 surrounds bellows 26 and a portion of contact rod22 and is rigidly secured as by brazing to plate 14. Sleeve 28mechanically supports a washer 30, preferably of copper, which is brazedas at 31 and crimped in place as at 33 near one end of the sleeve.Washer 30 has an aperture 32 for accommodating movable contact supportrod 22 and serves to support the rod against axial misalignment as itmoves back and forth.

In the vacuum switch hereinbefore described it is necessary to maintaina high vacuum and an uncontaminated environment to assure a minimum ofarcing and rapid arc extinguishment as the contacts 18 and 24 separateunder load. Most impurities are removed during various steps inmanufacture and assembly of the switch and substantially all impuritiesnormally in gaseous form are removed during the establishment of thevacuum within glass envelope 10. However, trace impurities still remainafter the tube is sealed and some impurities appear as a result of gasesreleased from the contact materials during arcing. Accordingly, meansare usually provided to getter the impurities which are present ingaseous form, such as hydrogen and methane.

Formerly, a small piece of silver-palladium alloy cut from sheet stockwas rigidly secured to one end of a small piece of wire as by brazingand the other end of the wire was secured as by brazing to a suitablemember within glass envelope 10 of the vacuum switch. However, such anassembly was rather delicate because of its necessarily small size andwas subject to detachment which created a risk of damage to or shortingof the vacuum switch.

In accodance with the present invention, it is proposed to provide amember already within glass envelope 10 of the vacuum switch andnormally serving another function with a silver-palladium plating toserve as a getter. In the embodiment shown washer 30 which supportsmovable contact support rod 22 is provided with a silver-pal ladiumplating to also serve as the getter in the following manner.

In the particular embodiment shown, only one side of washer 30 needs tobe plated since it has sufficient surface area with respect to theenvelope volume to absorb impurities which might be present. To plateonly one side of washer 30, the other side is coated with a substance toresist deposition of plating, such as Microstop lacquer.

Washer 30 which is, for example, made of copper is chemically cleaned bydipping in a suitable cleaning solution such as trichloroethylene ortetrachloroethylene in a still or ultrasonic bath. It is then thoroughlywater rinsed in cold running water before putting into a solution suchas 50 percent HNO by volume for ten seconds.

It is preferable at this stage to provide washer 30 with a prior silverstrike to improve the adhesion of the subsequent silver plating. This isaccomplished by passing a very high current, on the order of 40 to 50amperes per square foot, for 30 seconds while the washer is submerged ina weak silver plating solution. Washer 30 is then submerged in anaqueous silver cyanide platin bath and connected as a cathode with asource of silver for deposition submerged in the same bath and connectedas an anode. A plating current having a current denstiy of about amperesper square foot of the surface area to be plated is then passed throughthe cathode, bath and anode to deposit a plating of silver on washer 30.For each minute of plating time at the current density specified, alayer of silver millionths of an inch thick will be deposited on thesurface of washer 30. A layer of silver of an inch thick deposited after20 minutes on a washer about 2 /8 inches in diameter and having a hole 7inch in diameter was found to be suitable for use in a getter for avacuum switch rated at 15.5 kv. and 12,000 amperes.

After the silver is plated on washer 30, the washer is submerged in anaqueous plating bath comprising palladium chloride in a ratio of 50grams per liter of distilled water, ammonium chloride in a ratio of 50grams per liter of distilled water and an acid for maintaining theacidity of the bath below 0.5 pH. The tank for the bath is, for example,made of glass or plastic lined steel. Washer is connected as a cathodewith an anode made of platinum (such as Platanode) or stainless steel(such as 316 stainless steel) in the same bath. A suitable anode tocathode surface area ratio was found to be 2 to 1.

A plating current having a current density of about 4 amperes per squarefoot of the surface area to be plated is then passed through thecathode, bath and anode to deposit a plating of palladium on the platingof silver already on washer 30. During plating of the palladium theplating bath should be agitated vigorously as by means of a mechanicalimpeller. For each minute of plating time at the current densityspecified, a layer of palladium 12.5 millionths of an inch thick will bedeposited on the silver plated surface of washer 30. A layer ofpalladium 1 /2 thousandths of an inch thick deposited after eight hourson one side of washer 30 having the dimensions described was found to besuitable for use as a getter. In practice, then, a ratio of silver andpalladium of 1 to 3 is suitable.

Since the palladium plating on the silver plating of washer 30 isporous, the washer is then subjected to heat treatment to causediffusion of the silver into the palladium and alloying of them. Thus,the plated Washer is heated for eight hours at a temperature between 300and 400 C. Since the temperature at which washer 30 is heated is belowthe fusion points of the copper, the silver plating and the palladiumplating, it is apparent that solid state diffusion occurs to produce thedesired alloy rather than actual melting. Thus, desired tolerances canbe maintained by the method disclosed which may be a critical factor insome devices.

An alloy as specified hereinbefore (25% silver and palladium) hasexcellent properties as a getter when applied in the amount specified inthe vacuum switch described particularly in absorbing hydrogen.

Experiment has shown that by changing the ratio of silver to palladiumto 75 silver and 25% palladium (by weight) produces an alloy or platingwhich resembles silver in appearance and texture but is extremelytarnish resistant in that sulphides are not readily absorbed.Heretofore, it has been the practice to add chromium oxide or chromiumtrioxide to silver cyanide electroplating baths to plate out an alloy ofsilver chrome but the resultant product does not have the soft color ofsilver and tends to resemble chromium. Furthermore, this product has arelatively high electrical resistance and is not desirable as anelectrical conductor. Chromate conversion coatings are used to keepsilver from tarnishing before customer use but are very thin. A rhodiumflash coating over silver has antitarnishing qualities but has a hard,cold appearing surface which could tend to peel.

In accordance with the present invention an article is provided with 75%silver plate and 25% palladium thereover (by weight of coating) in themanner hereinbefore described. The resultant product is then heated foreight hours at between 300 to 400 C. to effect solid state diffusion andalloying between the silver and palladium.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A process for plating an article with a silver-palladium alloycomprising the steps of:

(a) submerging the article in a first bath containing a silver platingsolution and a suitable anode and passing a current of a density ofabout 40 to 50 amperes per square foot through said anode, first bathand article for about 30 seconds to provide the article with a silverstrike,

(b) submerging the article in a second bath containing a silver cyanidesolution and a second suitable anode and passing a current of a densityof about 10 amperes per square foot through said second anode, secondbath and article for about 20 minutes to provide the article with asilver plating,

(c) submerging the article in a third aqueous bath containing about 50grams of palladium chloride per liter of water and about 50 grams ofammonium chloride per liter of water and having an acidity of about 0 to0.5 pH and having a third suitable anode and passing a current of adensity of about 4 amperes per square foot through said third anode,third bath and article for about 8 hours to provide the article with aplating of palladium,

(d) agitating said third bath while plating is carried on therein,

(e) heating the article at a temperature of about 300 to 400 C. forabout 8 hours to alloy the silver and palladium thereon by effectingsolid state diffusion therebetween.

References Cited UNITED STATES PATENTS 923,864 6/1909 Levy 204-46 X3,150,065 9/1964 Fatzer 204-47 FOREIGN PATENTS 5,600 1895 Great Britain204-37 454,017 9/1936 Great Britain 204-19 GERALD L. KAPLAN, PrimaryExaminer W. I. SOLOMON, Assistant Examiner US. Cl. X.R. 20419; 313-174

